-- Investigators now do not think there was a second shooter, D.C. Police Chief Cathy Lanier said late Monday evening. Throughout Monday, authorities had run down witness reports and other evidence indicating there might have been additional gunmen.

-- The names, ages and a few biographical details are starting to emerge about the victims. As NPR's Hansi Lo Wang said on Morning Edition, of those named so far, they ranged in age from 46 to 73. Late Monday, police identified seven of those who were killed.

Michael Arnold, 59

Sylvia Frasier, 53

Kathy Gaarde, 62

John Roger Johnson, 73

Frank Kohler, 50

Vishnu Pandit, 61

Kenneth Bernard Proctor, 46

William Venable told Hansi that he was a friend and former co-worker of Johnson's. Every day, Venable said, Johnson would greet him with an enthusiastic "How 'ya doin' Buddy?!"

Of the other six besides Johnson that have been named by officials, the Post says that:

Arnold, a consultant who lived in Lorton, Va., "graduated from the U.S. Naval Academy and served in the Navy for 29 years, before retiring as a captain last year."

Frasier, from Maryland's Charles County, was a "network-security administrator with the Naval Sea Systems Command.

Gaarde, a financial analyst from Prince William County, Va., "was just the kindest lady in the world," says neighbor Patrick Bolton. "I'm not even exaggerating. I've never seen her do anything but nice things for people."

Kohler, Maryland's St. Mary's County, was married with two daughters.

Pandit was "a very nice man with an Irish setter," according to neighbor Mike Honig in North Potomac, Md.

Proctor, "a civilian utilities foreman" from Charles County, Md., was a "very loving, caring, gentle person," according to his former wife, Evelyn Proctor.

-- Many details remain unclear. While there are reports of 8 people being wounded, our colleagues at WAMU say "as many as 14 others" were injured.

-- As for the man who police say was responsible, 34-year-old Aaron Alexis, more continues to come out about him. WAMU sums up his biography this way:

"According to the U.S. Navy, Alexis, who hailed from New York, enlisted on May 5, 2007, reaching the rank of Aviation Electrician's Mate 3rd Class in 2009. He received both the National Defense Service Medal and the Global War on Terrorism Service Medal. The Ft. Worth Police Department arrested Alexis in September 2010 for improperly discharging his weapon into a neighbor's ceiling. (The incident report.) He was also apparently involved in another gun-related incident in Seattle in 2004."

As we reported Monday evening, "Alexis likely had a security clearance that gave him access to the Navy Yard" because he was employed by a defense contractor " to refresh computer systems used on the Navy Marine Corps Intranet."

We'll continue to follow developments and post updates.

Update at 4:16 p.m. ET. 'Hearing Voices':

We've updated our profile of Aaron Alexis with a police report obtained by NPR, which offers a narrative of a disturbing incident in which Alexis complained of "hearing voices."

Update at 2:47 p.m. ET. Background Check Revealed No Issues:

The Experts, the IT firm that employed Alexis for six months over the past year, says Alexis had a government security clearance and that they hired a service to perform two background checks.

"The latest background check and security clearance confirmation were in late June of 2013 and revealed no issues other than one minor traffic violation," the company said in a statement emailed to NPR.

Update at 2:35 p.m. ET. Shooting Lasted More Than 30 Mins:

During a televised press conference, D.C. Police Chief Cathy Lanier said "from start to finish" the shooting rampage lasted more than 30 minutes but no more than an hour.

Valerie Parlave, assistant director in charge of the FBI Washington Field Office, said that Alexis bought a shotgun legally in Virginia. Alexis, Parlave said, had been staying in the area since about Aug. 25.

Update at 12:40 p.m. ET. More Victims Identified.

The D.C. Police just reported that "five additional decedents have been identified as":

Update at 9:55 a.m. ET. Alexis Was Given Honorable Discharge, Official Says:

Monday, a Navy official told NPR that Alexis had been given a "general discharge" from the Navy in January 2011. That's a step below an "honorable" discharge. But Tuesday a "military official" told NPR's Tom Bowman that while the Navy had pursued a general discharge, Alexis was eventually given an honorable discharge after he applied for the Early Enlistment Transition Program. That program, the Navy says, "allowed approved sailors to voluntarily separate from the Navy prior to the end of their enlistment."